Metal-to-metal seals are very desirable in high pressure valves which are subjected to erosion, such as in kelly valves used to control blow outs in a drill string. A typical design for a high pressure valve of this type uses a spherical valve ball sealing against a valve seat which is concave in the sealing area. As can be imagined, these type valves must be made to close tolerances because they will not otherwise seal.
There are several factors that are pertinent in a valve ball--concave seat arrangement: (1) the roundness of the ball, (2) the radius matching between the ball and seat and (3) the nature of the surface finish. The roundness of the ball is also related to the problem of rotating the ball on its centerline. It will be appreciated that if the ball is perfectly round, it has an infinite number of centerlines and can be rotated about any of them. If the ball is quite out of round, it must be rotated substantially only on one of its centerlines, e.g. if the valve ball is football shaped, it can only be rotated on a small number of axes, probably only two.
With valve ball--concave seat arrangements having acceptable metal-to-metal seals, the ball roundness must be in the range of + or -0.0005" and the radius matching must be nearly perfect. Tolerances in this range are beyond the capability of state of the art metal cutting machinery such as computer controlled machining centers. Thus, these type valves are presently made by machining the ball and valve seat to as close a tolerance as possible and the parts are then lapped.
Lapping is a machining operation in which the parts are placed in fixtures and abutted together so they can be moved in the same manner as in use. An abrasive paste is applied to the parts and the parts are then rubbed together. The parts are rubbed together until the metal is worn off one or both parts to an extent where the parts meet the desired tolerance. This is a lengthy and expensive process so high pressure ball valves with metal-to-metal seals are expensive. When the time comes to rebuild a valve, one must replace the valve ball and the seat because each valve ball is made to fit its seat. Thus, modern high pressure metal-to-metal sealed ball valves are not mass produced if that term means that parts of identical devices are interchangeable.
Whether a valve ball has been lapped can be determined by examining it under magnification. Machining produces arcuate striations that are visible under magnification. When a valve ball is lapped, the visible striations are smaller and are typically cross-hatched. Interestingly, coating a valve ball does not disguise imperfections in the underlying surface, it magnifies them. Thus, inspection of a coated or uncoated valve ball under magnification shows whether it is lapped or untapped.
A disclosure of a ball valve having a metal convex valve seat is found in U.S. Pat. No. 3,888,460. Other disclosures of interest are found in U.S. Pat. Nos. 2,932,311; 2,988,109; 3,077,895; 3,185,857; 3,211,421; 3,272,472; 3,380,709; 3,386,461; 3,556,471; 3,610,575; 3,705,707; 4,813,649 and 5,632,294.